Was just thinking it would be cool to have a harder version of the treasure maps.
They would be purple and titled something different, like 'Ancient treasure map' or 'Pirate Lord's treasure map' or 'Dweomer's treasure map'. The map might have multiple locations you have to travel to before actually finding it, or perhaps they all end in 'dangerous' areas. Delves or dungeons, maybe.
Each chest would also have a guardian or challenge of some type to overcome - I was thinking it should possibly be challenging enough to be group content. Something on the level of a dolmen or perhaps a weaker world boss. You dig up the 'Ancient' map and a bunch of ghostly warriors spawn to protect it. Digging up the 'Pirate lord's' chest would cause the group of pirates that has been 'trailing you' to finally attack. The Dweomer map could lead you into a Dweomer ruin, with a very large construct of some sort protecting the chest.
The chests, of course, would be lootable by the group and have slightly better treasure than a standard chest.
I think hunting down treasure chests as a group could be a lot of fun